A CPP-GMR head is considered as one promising sensor to replace the conventional CIP (current in plane) GMR head for over 200 Gb/in2 recording density. In a typical CPP-GMR sensor, a bottom synthetic spin valve film stack is employed for biasing reasons and a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP-GMR technology. GMR spin valve stacks are known to have a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer (spacer). One type of CPP-GMR sensor is called a metallic CPP-GMR that can be represented by the following configuration in which the spacer is a copper layer: Seed/AFM/AP2/Ru/AP1/Cu/free layer/capping layer. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The pinned layer may have a synthetic anti-parallel (SyAP) structure wherein an outer AP2 layer is separated from an inner AP1 layer by a coupling layer such as Ru. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, in the CIP sensor, the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher for a CPP configuration. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity.
Another type of sensor is a so-called confining current path (CCP) CPP GMR sensor where the current through the Cu spacer is limited by the means of segregating metal path and oxide formation. With a current confining (CCP) scheme, CPP GMR performance can be further improved. An example of a CCP CPP-GMR sensor has the following configuration: Seed/AFM/AP2/Ru/AP1/Cu/CCP layer/Cu/free layer/capping layer where the CCP layer is sandwiched between two copper layers.
In a CPP operation mode, a tunnel magnetoresistive (TMR) head is another candidate for realizing high sensitivity. In this design, the non-magnetic conductive layer between the pinned layer and free layer in the GMR stack is replaced by an insulating layer such as AlOx or MgO. When the magnetoresistive element is a magnetic tunnel junction (MTJ), the tunneling (insulating) layer may be thinned to give a very low RA (<5 ohms-μm2).
A MTJ element in a magnetic device such as a read head may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The bottom layer in the MTJ element is typically comprised of a seed layer such as NiFeCr or a Ta/NiCr composite which promotes a <111> lattice orientation in overlying layers. However, in a related Headway patent application Ser No. 11/652,740 which is herein incorporated by reference in its entirety, a Ta/Ru or Ta/Hf/NiFe seed layer may be employed to improve the Hex/Hc ratio. Generally, an antiferromagnetic (AFM) layer, ferromagnetic “pinned” layer, tunnel barrier layer, ferromagnetic “free layer”, and a capping layer are successively formed on the seed layer to complete the MTJ stack. The pinned layer has a magnetic moment that is fixed in the “x” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “x” direction. The thin tunnel barrier layer above the pinned layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed through the MTJ stack in a direction perpendicular to the layers therein, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
One indication of good device performance is a high magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the maximum change in resistance observed by changing the magnetic state of the free layer. In order to achieve desirable properties such as a specific junction resistance x area (RA) value, a high dR/R value, and a high breakdown voltage (Vb), it is necessary to have a smooth tunnel barrier layer that is promoted by a smooth and densely packed growth, such as a <111> texture for the AFM layer, pinned layer, and seed layer. Although a high RA value of about 10000 ohm-μm2 is acceptable for a large area (A), RA should be relatively small (<1000 ohm-μm2) for smaller areas. Otherwise, R would be too high to match the resistivity of the transistor which is connected to the MTJ. Other desirable magnetic properties for an MTJ are a small interlayer coupling field (Hin) between the pinned layer and free layer, and a strong exchange coupling field (Hex) between the AFM layer and pinned layer is important to maintain the pinned layer magnetization in a certain direction.
For better head performance, it is always desirable to have a larger Hex to ensure pinning robustness between the AFM and AP2 layers and a smaller pinning dispersion. In other words, a larger Hex/Hc ratio is needed to suppress pinning field related noise. Improvement in the exchange bias properties can be achieved by proper selection of the seed layer, AFM layer, and pinned layer. However, further optimization than heretofore achieved is necessary for ultra-high density recording heads.
During a routine search of the prior art, the following references were found. In U.S. Patent Application 2006/0061915, a 5 Angstrom thick CoFe layer is inserted between the seed layer and the AFM layer in a MTJ stack to increase Hex and the Hex/Hc ratio.
U.S. Patent Application 2006/0056114 describes the use of an oxide layer insertion into the pinned layer and adjacent to the AFM layer to prevent Mn diffusion from the AFM layer into the tunnel barrier layer. The composite magnetic layer may have an amorphous phase and a crystalline phase.
In U.S. Pat. No. 6,801,414, an amorphous layer is inserted into a pinned layer to suppress Mn diffusion from the AFM layer. The amorphous layer has a composition MX where X is an oxide, nitride, or carbide, and M may be Ti, Ta, V, Al, Sc, or Eu.
U.S. Pat. No. 6,718,621 discloses a pinned layer that may be comprised of an amorphous material such as CoFeB.
U.S. Pat. No. 7,063,904 describes a MTJ having a pinned layer and an AFM layer containing Cr so that the exchange coupling field (Hex) between the two layers is effectively increased. Preferably, the Cr concentration decreases with distance away from the interface between the two layers.